The Secret History of Invisible Ink, Part 1

When I was a kid, I could write in invisible ink – really! Take some lemon juice, apply to paper with a brush or stick, and then heat over the light bulb in one of our living room lamps. Voila! Brown lettering would appear on a blank white sheet of paper. A bit unsophisticated perhaps, but it worked. I didn’t really have any secrets to smuggle past the authorities – it was the mere fact of writing invisibly that made it so cool.

The CIA describes these timeworn items succinctly and well: “One document outlines the chemicals and techniques necessary for developing certain types of secret writing ink and a method for opening sealed letters without detection. Another memorandum dated June 14, 1918 – written in French – reveals the formula used for German secret ink.” My favorite is document number 6: Invisible Photography and Writing, Sympathetic Ink, Etc., a four-page pamphlet compiled by Theodore Kytka, identified as “Handwriting Expert, San Francisco, Cal.” and “printed by the San Francisco Division [of what, I wonder? It doesn’t say] for the information of Post Office Inspectors.”

According to the CIA, only recently have advances in technology made these various formulae obsolete, spy-wise. Among the secrets: “A German Formula. Take one ounce of alum and one ounce of white garlic juice. Write with a quill and on heating the paper the letters become very legible and cannot be removed by salt water application.” Then there’s “Disappearing Ink. Take a weak solution of starch, tinged with a little tincture of iodine. The bluish writing will soon fade away.” Boy, is my mother lucky I didn’t latch onto this information – our kitchen would have been a disaster area!

Not only is this a really great story, it also ties into GPO’s 150th anniversary in a very interesting way. During World War II, our scientists helped to thwart the use of invisible inks, like those described above, by Axis prisoners of war. In my next post, I’ll tell that story. Stay tuned!

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18 Responses to The Secret History of Invisible Ink, Part 1

Invisible ink has been used for thousands of years since the Roman empire. They used things like milk that would not be activated until it was held near the heat of a candle flame. Did you know that they even wrote secret messages on the inside of eggs by using a special compound that transferred through the egg shell but did not show up on the shell itself. The message was clearly written on the hard boiled egg and could be deciphered after it was peeled. I just thought it was something interesting and relevant to share here, and thank you for providing us some history on it as well.

Hello There. I found your blog using msn. This is an extremely well written article. I’ll make sure to bookmark it and come back to read more of your useful information. Thanks for the post. I will certainly return.

Oh what fun! This brings me back to my elementary school days, and we would write with lemon juice, and watch it disappear. It also brings to mind, the current usage of invisible ink, in crime prevention. I am the Neighborhood Watch Captain of my subdivision, and I have a pen which our local crime prevention organization sold me for $2.00 which marks appliances, automobiles, and valuable item with an invisible ink, seen only under a black light (a small black light flashlight is given to all law enforcement personnel), which they recommend you write your Driver’s License number (not your SSN). Thanks for the opportuinity to spread the word about the importance of invisible ink.

Ah those were the days most boys are the same,I recall the two main things were invisable ink and —now this is going to date me,getting the correct proportions of S, S and Willow Charcoal.I remember getting it right, scaring myself and friends and never doing it again.
I look forward to you next report on the ink.
Alan